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More than Chicken Feed: Milling Maize for Food Products Part 2

March 01, 1997
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by Teresa Acklin

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Part 2 of 2 discusses technologies used in the modern maize mill.

By Mario Cinquetti

A modern maize mill is characterized by one of three degermination systems; the choice depends which will give the most economical results for the products made.

The three degermination methods consist of the dry system, which uses impact degerminators; the partially wet system that employs polygonal deger-minators with blades; and the partially wet system that uses friction or conical degerminators.

The dry degermination system is designed to obtain flour and semolina for polenta and flour for bread, pasta and biscuits; germ with 21% to 23% oil on a dry matter basis; and small percentages of brewery grits or grits for extrusion with a fat content of less than 1%, dry matter. Mills with a dry degermination system have an electric power consumption of 45 kilowatts per tonne and do not need to dry the finished products.

The partial wet degermination system using polygonal deger-minators with blades offers the highest yields of brewery grits at 56% to 58%, of snack food grits at 51% to 52% with less than 0.9% fat and of germ with fat of 18% to 19%. The power consumption of mills using polygonal deger-minators is about 60 kW per tonne of processed maize.

Unfortunately, when processing maize dampened to 17.5% to 18% moisture, the average moisture level in the germ and meal is about 17%, and in grits, it is about 14.5%. These products then must be dried, which requires considerable heat consumption of about 10 kilograms of gas or oil per tonne of processed maize.

The third system, partial wet degermination with friction degerminators, provides the best results in dehulling of the caryopsis, but lower yields of brewery or extrusion grits. The system also has the highest power and drying heat requirements, at 65 kW per tonne and 12 kg of gas or oil per tonne, respectively.

This system is used for the production of large, completely dehulled and degermed grits, often called "hominy grits," with granulometry ranging from 3,200 to 5,600 microns. This type of grits is used to produce "corn flakes" breakfast cereals.

Dry Degermination

Maize is degerminated by impact centrifugal degerminators. The product thus obtained is separated, based on its granulometry, by the plansifter and by conical turbo aspirators through air current; and, based on specific weight differences, by gravimetric separators.

In a 140 tonne per day maize milling plant equipped with the dry degermination system, the degermination section can consist of two impact degerminators, two conical turbo aspirators, one vertical turbo aspirator and two gravimetric separators.

After the cleaning operation, carried out by one separator, one aspirator and one magnetic apparatus followed by a dry stoner, the maize is dampened with 1.5% to 2% water and allowed to temper for eight hours. Then it is sent to the degerminators.

Through impact action, the deger-minators detach the germ from the caryopsis. This is possible because of the fragility of the vitreous part, the friability of the floury fraction and the elasticity of the germ. The rotor speed is 720 revolutions per minute for U.S. No. 3 yellow dent maize.

The product then is sent to the plansifter, which separates it in fractions of different sizes. The badly broken grains are sent back to the degerminators, passing through a vertical aspirator, to separate the small cob pieces.

The bigger grits of the vitreous fraction with the germ pass through conical turbo aspirators to separate the bran and dusts, which will be separated from the air by a filter. These bigger grits then are conveyed to the first gravity separator, while the medium grits with germ are sent to the second gravity separator.

The gravity separators split the product in two fractions: the heavy fraction, about 70%, will be sent to the rollermill for refining.

The light fraction, about 30%, contains 99% of the germ and will be sent to another rollermill after being dampened with 0.5% water. This rollermill has wide and flat flutes for flaking the germ particles, which will be easily separated by the plansifter.

The maize grits, once separated from the germ, will be refined by the rollermills and sized by the plansifter and purifiers to obtain the flours and grits with the required granulometry and purity.

A dry degermination plant with a capacity of 140 tonnes per 24 hours, processing cleaned U.S. No. 3 yellow maize, can produce up to 45% brewery grits with 1% fat content, up to 75% to 80% polenta meal, or 60% fine bakery flour.

This type of maize mill can be installed on a metallic structure with an area of about 200 square meters requiring a gutter height of 13 meters, accounting for the daily bins, tempering bins and finished products silos.

The impact degerminator requires a 18.5 kW motor. The total power consumption to produce brewery grits or polenta meal will be 45 kW per tonne of maize, which will increase by 5 kW per tonne if producing 60% bakery flour.

The main advantages of a dry degermination system are:

• no power consumption or thermic power consumption to dry the finished products since moisture added in the process evaporates during the eight-hour tempering time, degermination and refining action;

• limited power consumption during the milling process;

• easy maintenance;

• minimal space requirements per tonne of capacity.

Partially Wet Degermination

With the bladed wet system, maize previously dampened at 17.5% to 18% is partially dehulled and degerminated by polygonal degerminators; these use an octagonal rotor with eight sides that are fitted with 160 inclined blades with a special cutting profile.

This is the ideal system for U.S. No. 3 dent maize to produce brewery grits with the highest yields (56% to 58%) and minimum fat content (less than 0.9%, dry matter) or to produce grits for extrusion (50% to 52%). The drawback is that the system requires drying the germ and animal meal, which are produced with a moisture of 17%; grits, which are produced with a moisture 14.5%, also may require drying. Moreover, the germ oil content can vary from 17% to 18% versus the 21% to 23% yield obtained with the dry system.

With this system, conditioning water is added at 2.5% to the maize, which is tempered for eight hours to make the germ soft and elastic. A final tempering with the addition of 3% to 4% water and a 15- to 20-minute tempering time allows the bran to partially detach from the endosperm, facilitating the action of the degerminators.

The polygonal degerminator is essentially a cylindrical stator, made of half checkered plate and half manganese steel perforated sheet. The interior is fitted with a set of retaining baffles, adjustable from the outside with the machine running, and by an octagonal rotor with 160 blades.

The tempered cereal is distributed on the rotor action area, between blades and stator. It is rubbed vigorously by the rough surfaces of the stator and of the retaining baffles, in addition to intense self-rubbing action. With yellow dent maize, the rotor speed is about 730 r.p.m.; the dehulling effect can be easily adjusted by setting the inclination of the retaining baffles.

After passing through the two degerminators, the product is separated in two fractions by a turbosifter suitable to process very moistened products: the fine fraction is dried immediately by a thermopneumatic unit and then separated by one turbosifter; the heavy fraction, after being aspirated by a turboas-pirator, is sieved by a plansifter, which feeds the two gravimetric separators after an adequate aspiration carried out by two turboaspirators.

The gravimetric separators split the broken grains in two fractions: the heavy one, representing about 70%, is sent to the clean refining line; the light fraction, representing about 30% containing 99% of the germ, is conveyed, after previous dampening (0.5% water), to the "dirty" refining line. The refining line's rollermill uses rolls with large and flat flutes, which flake the particles of germ successively separated by the plansifter.

The degermination department mainly consists of two polygonal degermi-nators, two turbosifters, two conical turboaspirators, one vertical aspirator and two gravimetric separators.

Refining operations for grits are carried out by four rollermills, one plansifter with eight channels and 30 sieves per channel, two multiple vibrating purifiers, three conical turboaspira-tors and two vertical aspirators, one turbosifter and one vertical vibrosifter. This type of plant can be installed in a three story building with dimensions of 16 by 7 by 24 meters.

The polygonal degerminator requires a 45 kW motor; the total power consumption to produce mainly brewery grits or extrusion grits will be 55 kW/t. Power consumption is 60 kW to produce 60% of fine flours for bread, pasta or biscuits; in the latter case, a quadruple rollermill should be added. The gas/oil consumption to dry the germ, animal meal and grits is about 10 to 11 kg per tonne of processed maize, depending on the vegetal moisture of the maize.

The tables on page 18 compare yields on dry and partially wet degermination systems for two end products. The results indicate the dry degermination system is the choice for basic production of fine flours for bread, pasta, biscuits and polenta meal. This type of plant alternately enables the production of brewery grits and extrusion grits at lower yield.

Partially wet degermination is advisable when we want to produce the highest percentage of brewery grits or extrusion grits, particularly in markets where demand for polenta meal is not strong.

In both cases, it is convenient to couple the maize mill to a feed mill and an oil mill.

This article is based on a presentation by Mario Cinquetti, sales director with Braibanti Golfetto, Padova, Italy, at the 8th annual conference and trade show of the Association of Operative Millers, Middle East/East Africa District 16, in Luxor, Egypt, in November.